1. Field of the Invention
This invention relates to a torsion bar-type stabilizer device used in a suspension system of a motor vehicle and, more particularly, to a stabilizer device capable of controlling the torsional rigidity of a torsion bar according to vehicle driving conditions.
2. Description of the Prior Art
In the suspension system of a motor vehicle, there is commonly used a stabilizer device which uses a torsion bar having a spring-action section extending in the direction of width of vehicle and rotatably supported on the vehicle body side by a part of the torsion bar, and a mounting level portion bent, at both ends of the spring-action section, in the longitudinal direction of the vehicle, and secured at a free end thereof on the right and left wheel sides. The use of this stabilizer device improves the rigidity of the suspension system against the rolling of the vehicle by utilizing the elastic force of the torsion bar produced by torsion that has occurred in the spring-action section of the torsion bar when the right and left wheels make an up-and-down motion, thus achieving vehicle driving stability.
In high-speed driving vehicles or off-road driving vehicles, it is desirable to increase the spring constant of the torsion bar for the purpose of improving the rolling resistance of the vehicle. If, however, the spring constant is increased, the right and left wheels are likely to make incessant anti-phase up-and-down motion, resulting in lowered driving comfort. To cope with this drawback, there has been proposed a stabilizer device (for example in Laid-Open Japanese Utility Model Registration Application [KOKAI] No. 63-40214 and Laid-Open Japanese Patent Application [KOKAI] No. 63-137009), in which the stabilizer device mounting section on the wheel side or on the vehicle body side comprises a cylinder provided on the torsion bar or on either of a member on the wheel side or a member on the vehicle body side, a piston rod of a piston secured on the other side and the piston separating the interior of the cylinder into two chambers; openings provided in the wall of the cylinder and located in positions corresponding to a neutral position of the piston in the cylinder and to the ends of the two cylinder chambers; a closed circuit formed for connecting the fluid passage communicating with these openings to an accumulator, and filled with a fluid; and a solenoid valve or an operation check valve provided in a passage connecting the fluid passage to the two cylinder chambers, and controllable from the driver's seat. When the solenoid valve or the operation check valve is operated to allow the free passage of the fluid between the two cylinder chambers, the piston makes a free relative motion relative to the cylinder if the up-and-down motion of the wheels is transmitted to the piston rod, and accordingly the torsion resulting from the up-and-down motion of the wheels will not occur in the spring-action section, thus making ineffective the action of the stabilizer device. When the solenoid valve is operated to stop the passage of the fluid, or when the operation check valve is operated to allow the unidirectional flow of the fluid, the movement of the fluid between the two cylinder chambers is checked, preventing the relative movement of the piston relative to the cylinder and accordingly causing a torsion resulting from the up-and-down motion of the wheels to arise in the spring-action section and an anti-roll affect by the torsion bar to occur.
In these devices, the cylinder has an opening for communicating one of the cylinder chambers with a fluid passage having an opening in a position corresponding to the neutral position of the piston for communicating one of the cylinder chambers with an accumulator through a fluid passage. The fluid in the cylinder chamber flows into the accumulator through the opening if the piston stops in an arbitrary position when the operation check valve or the solenoid valve is operated for locking the piston to prevent the free relative movement of the piston relative to the cylinder. Into the other cylinder chamber, the fluid is admitted to flow by the operation check valve functioning as a check valve, or by a substitute check valve, which allows the movement of the piston to the neutral position. When the piston arrives at its neutral position, the piston closes the opening in the cylinder corresponding to the neutral position in the cylinder chamber. At this time, the relative movement of the piston relative to the cylinder is fully stopped, thereby exerting the torsional rigidity of the torsion bar during anti-roll action equally to both the right and left wheels.
According to the prior-art stabilizer device described above, the cylinder is provided with an opening communicating with the accumulator, in a position corresponding to the neutral position of the piston, in order to hold the piston in the neutral position in the cylinder during the anti-roll action of the stabilizer device. In this device of above-described constitution, the piston is allowed to move to the neutral position regardless of the present piston position at the time when the movement of the fluid between the two cylinder chambers has been checked by the solenoid valve or the operation check valve. When the piston closes the opening located in a position corresponding to the neutral position of the piston, the movement of the fluid is checked to lock the piston, thus preventing the relative movement of the piston in relation to the cylinder.
For closing the opening with the piston itself, either in a device having a reciprocating piston in which piston rod moves in the axial direction of the cylinder or in a device having a rotary piston in which a piston rod turns around the center of axis of the cylinder, it is not easy to maintain fluid-tightness between the piston and the cylinder and accordingly it is very difficult to hold the piston in the locked position. For example, the use of a piston seal produced of a soft elastic material presents an increased danger of shearing of the piston seal by an edge of the opening. Also, the use of a piston seal of rigid synthetic resin or metal produces a gap between the piston and the cylinder inner wall, through which the fluid will leak, resulting in a failure of piston locking.